An air conditioning system including a refrigerant circuit wherein a compressor, a heat source-side heat exchanger, a pressure-reducing valve, and a utilization end-side heat exchanger are sequentially connected and wherein its refrigerating cycle is switchable between forward cycle and reverse cycle, has been known which, as disclosed in, for example, Japanese Utility Model Application Laid-Open No. 63-15434, can perform a so-called reverse cycle defrost operation such that when, during a heating operation, frosting occurs at the heat-source-side heat exchanger, the refrigerant circuit, upon receipt of a defrost command, is operative to switch the refrigerating cycle to a cooling cycle so that a discharge gas refrigerant (hot gas) is flowed into the heat source-side heat exchanger for a predetermined time or until the temperature at the heat-source-side heat exchanger rises to more than a predetermined temperature value, whereby the frost at the heat-source-side heat exchanger is melted for restoring the capability of the heat exchanger.
In such an air conditioning system, when, at the beginning or end of a defrost operation, the refrigerating cycle is forwardly or reversely changed, liquid refrigerant begins to flow toward the compressor as a result of a functional change to an evaporator of the heat-source-side heat exchanger or utilization-side heat exchanger, which has been functioning as a condenser and which, therefore, has a large amount of liquid refrigerant stored therein. In such conventional type of air conditioning system, therefore, an accumulator is disposed before the compressor to absorb the liquid refrigerant thereby to prevent liquid back-flow to the compressor.
With the accumulator so disposed, however, the system may involve various troubles including a power decrease due to pressure reduction, and separation of oil and liquid refrigerant into two phases. Essentially, therefore, an accumulatorless arrangement is desired.